This invention relates to multilamp photoflash units and, more particularly, photographic flashlamp units having readily visible means for indicating a flashed lamp.
One type of previously known flashed lamp indicator is described in U.S. Pat. No. 3,718,815 (Mc Donough et al) as applied to a flash cube. This indicator is a two-color type comprising an insoluble pigment of the desired initial color, a quantity of powdered oil-soluble dye of the desired final color, and a powdered organic additive which melted and dissolved the dye upon flashing of the lamp associated therewith. The second color, e.g., red, thereby masked the initial color, e.g., light blue. The three powdered ingredients were applied from an aqueous slurry which included a small amount of water soluble binding agent. The spot or mark was applied by a transfer or printing process and required drying after application. Although quite useful, this prior indication method exhibited several weaknesses. For example, the spot could become discolored if the product was stored for a long period at high temperatures, such as in an automobile glove compartment during the summer. The printing and drying process is not sufficiently trouble-free to be readily incorporated into high-speed, automated equipment because the paste is affected by excessive working or shear-continuous milling action between a roller and platen which effects too intimate contact between the particles of dye and organic meltable solvent, thereby again causing discoloration. Further, the initial color must be muted so as to be overshadowed by the final color, in which the particles of initial pigment remain.
Another type of flash indicator that has been used commercially employs a heat-shrinkable thermoplastic polymer such as biaxially oriented polypropylene. For example, one type of commercially available flash unit that employs such an indicator comprises a bidirectional linear array of lamps and is referred to as a "flash bar". An indicia sheet insert is located within the flash unit container above the lamps. Heat-shrinkable colored plastic flm comprising the indicator material is laminated to the under side of the indicia sheet, which has aperture aligned with each lamp to expose the brightly colored indicator film. The lamps of the flash unit are assembled in reflectors which have respective openings for providing thermal access paths to the respective aperture-exposed areas of indicator material. Upon flashing of a lamp, the heat radiated therefrom causes the colored (e.g. blue) plastic film to shrink away and disappear from the respective indicia sheet aperture to leave a void, or dark spot, identifying the lamp as having been flashed. A variation of this indicating method, wherein the flash indicator aperture are located along the side walls of the flash bar, is described in U.S. Pat. No. 3,956,625 (Anderson). Application of this approach to an indicia sheet on the back of a planar array of lamps that is commercially available and referred to as a "flip flash" is described in U.S. Pat. No. 3,894,226 (Hanson).
The heat-shrinkable film indicators provide significant advantages over the previously mentioned dye-change indicator. For example, since the functioning temperature of the film is relatively high, it exhibits the ability to tolerate elevated temperature storage without partially changing color. Further, the plastic film indicator is readily adaptable to high speed automated equipment. Nevertheless, this single-color, disappearing film approach does have some drawbacks. For example, under conditions of poor ambient lighting, the open areas would appear black, and ambiguous user interpretations could result. Further, in multiple lamp array type flash units, such as the flash bar and flip flash, where a series of lamps in one section of the array are to be flashed in sequence, and then the unit is turned around or flipped over to activate a new section of lamps, there is no uniquely striking indication when all of the lamps in sequence have been flashed. The only indication is a plurality of void areas for the respective flash indicator apertures of this sequence. It is not an uncommon experience to users of flash bar and flip flash units to forget to reposition the flash unit when all the lamps in the sequence are used, thereby leading to wasted film.